IBM corporation has marketed several models of resistive ribbon thermal transfer printers under the trademark Quietwriter which print a single line of "letter quality" characters (i.e., with a minimum vertical resolution of 240 vertical pixels per inch (25.4 mm) and a minimum horizontal resolution of 360 pixels per inch (25.4 mm)) as a printhead is driven horizontally across the page at a constant speed. The printhead is provided with 40 electrodes arranged one above another in a vertical row. A special thermal transfer resistive ribbon having a resistive layer, a conductive layer, an ink release layer and an ink layer is positioned between the printhead and the paper (or other image carrier) on which the characters are to be printed, with the resistive layer in contact with the printhead and the ink layer in contact with the paper. Each electrode may be selectively activated and when activated causes a localized current to flow through the resistive layer to the conductive layer (which is grounded) thereby heating a small area of the ink layer in its immediate vicinity and causing a dot of ink to be released from the ribbon onto the image carrier. For best print quality, the thermal transfer ribbon should remain stationary with respect to the paper as the current is applied, and should be rapidly peeled away shortly thereafter at a predetermined angle between the ribbon and the printhead; the ribbon motion must be synchronized with the movement of the printhead. Accordingly, it is therefore conventional to print in only one direction, resulting in an interruption of printing whenever the printhead is being returned from the end of one line to the start of the next line. In the prior art printers used with the resistive ribbon thermal transfer process, only one line of characters is printed during a single pass of the printhead over the ribbon and the thus-used portion of the ribbon cannot be reused; fractions and other formulas occupying more than a single line and text employing subscripted and superscripted characters slightly below or above the printline are printed in several passes. Furthermore, because certain electrodes are used more than others, the high currents used in the process cause a differential erosion of the printhead electrodes, so that after a period of use the printhead no longer makes the required uniform ohmic contact with the resistive layer and no longer provides a uniform pressure on the ink layer as it is being released onto the paper.
The printhead, print ribbon, drive electronics and other major components for such a printer, including many process considerations, are described in detail in pages 443-477 and 494-538 of the IBM Journal of Research and Development Vol 29, Number 5, dated September 1985, and in U.S. Pat. Nos. 4,103,066 ("Ribbon Substrate"); 4,345,845 ("Drive Electronics"); 4,350,499 ("Resistive Ribbon Printing Apparatus and Method"); 4,400,100 ("Ribbon Layers"); and 4,456,915 ("Print Head"), which are hereby incorporated by reference.
It is to be noted that, although the thermal transfer resistive ribbon process produces printing of high quality, the prior art printers designed for use with that process are relatively slow and wasteful of expensive supplies.
A dot matrix printer capable of printing more than a single line of fixed height draft quality characters in a single horizontal sweep of the printhead is known from published European Patent Application No. A2 0 031 421 in the name of IBM and entitled MULTIPLE MODE PRINTING SYSTEM AND METHOD. Such a prior art printer is clearly unsuitable for use with the above described thermal transfer resistive ribbon process.